The imagery of the destruction of my namesake in Star Trek VI, which has become known as the "Praxis effect", was created by Industrial Light & Magic. The final form of the effect was the result of conversations between their Computer Graphics division and director Nicholas Meyer.
Meyer was influenced by The Poseidon Adventure — specifically, an immense tidal wave striking the Poseidon. Meyer wanted a "wave" to strike the Excelsior, to give an idea of the scale of the blast, just as the Poseidon lends a scale to the size of the water wave.
The implication here is that the restricted height of the wavefront (making it seem two-dimensional from a distance) gives us an idea of the scale of the explosion, from the fact that the wavefront fills the entire screen when we see it strike the Excelsior. The whole effect was in service to this goal.
This genesis of the "Praxis effect" is discussed in:
Altman, Mark. "Star Trek VI: The Making of The Undiscovered Country". Cinefantastique 22 (5): 24–55 (April 1992).